1. Field of the Invention
The present invention relates to semiconductor light emitting devices and, in particular, to resonant cavity light emitting devices.
2. Description of Related Art
Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III–V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Such devices typically have a light emitting or active region sandwiched between a p-doped region and an n-doped region. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxial techniques.
Semiconductor light emitting devices may be included in a variety of applications including displays such as flat panel displays, indicator lights such as traffic lights, and optical communication applications. In many applications, such as displays, it is desirable to have light emitted in a preferred direction. However, light from such semiconductor devices is typically emitted isotropically from the active region.
One method to improve the light emission characteristics of a device by providing more directed, anisotropic emission is proposed in U.S. Pat. No. 5,226,053, which teaches forming an optical cavity of an LED within a resonant Fabry-Perot cavity. FIG. 9 illustrates a resonant cavity LED (RCLED) according to U.S. Pat. No. 5,226,053. RCLED 110 comprises a bottom electrode 111, a substrate 112, a quarter-wave stack of a plurality of pairs of semiconductor layers forming a bottom distributed Bragg reflector (DBR) mirror, 113, one layer of each pair having a refractive index different from the refractive index of the other layer of the pair; a bottom confining layer, 114; an active layer or region, 115; a top confining layer, 116; a highly-doped contact layer, 117, and a top electrode, 118, having a centrally located aperture 119. The top mirror of the Fabry-Perot cavity is formed by an interface between contact layer 117 and air within aperture 119. Such a mirror has a reflectivity of the order of 0.25 to 0.35. The light emission takes place through the aperture. U.S. Pat. No. 5,226,053 teaches that the use of a Fabry-Perot resonant cavity formed by the DBR and the contact layer/air interface results in spontaneous light emission from the active region, which is restricted to the modes of the cavity.